Sag and fall, having eaten nothing (5)
I believe the answer is:
droop
'sag' is the definition.
('droop' can be a synonym of 'sag')
'fall having eaten nothing' is the wordplay.
'fall' becomes 'drop' (dropping is a kind of falling).
'having eaten' is an insertion indicator.
'nothing' becomes 'o' (looks like zero - 0).
'drop' going around 'o' is 'DROOP'.
'and' acts as a link.
(Other definitions for droop that I've seen before include "sink" , "lose heart" , "Sag, wilt" , "Go limp" , "Flag" .)